Avoid repeated Element dependency graph traversal

`Element.__update_cache_keys()` may traverse the same `BUILD` dependency graph multiple times while calculating weak and strict cache keys.
PR #2172 reduces this locally by materializing the dependency list once and reusing it for both calculations, but `Element._dependencies()` could be cached with something like [`@functools.cache`](https://docs.python.org/3/library/functools.html#functools.cache).

Start with Element.__update_cache_keys() and Element._dependencies(), then compare the local materialization approach in PR #2172 with Python's functools.cache documentation. Verify that the dependency graph is traversed only once while both weak and strict cache keys are calculated, without changing the resulting keys.

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